1. Field of Invention
The present invention relates generally to systems and methods for biometric identification, and more particularly, to imaging systems and methods capable of biometric identification according to more than one modality, especially for subjects positioned at a long distance from the image capture system.
2. Description of the Related Art
Humans possess multiple biometric characteristics which may be used for identification and access control applications. A criticism of existing biometric technologies has been that biometric acquisition procedures are often cumbersome and intrusive. For example, fingerprint biometrics offers an accurate solution, but acquisition of fingerprints typically requires contact (or near contact) with finger-print sensors. As such, finger-print biometrics is generally not scalable in terms of acquisition parameters, such as stand-off distance and capture volume. On the other hand, facial biometrics is advantageous because the face is generally in plain view and thus acquisition of corresponding biometric data tends to suffer from fewer constraints and may be less intrusive. However, facial biometrics is less accurate than other techniques, such as fingerprint biometrics.
As a result, an active area of research and development aims to improve biometric system design to facilitate acquisition and reduce intrusiveness. In particular, iris biometrics has spawned a great deal of research and development in recent years. Iris biometrics, however, has not yet become ubiquitous as compared to biometrics based on the face or finger-prints. Iris biometrics may be less intrusive than fingerprint biometrics and has been shown to be one of the most accurate biometrics, even exceeding the accuracy of finger-print biometrics. However, due to the smaller dimensions of the iris, acquisition of iris biometrics from a distance imposes more design challenges than the acquisition of facial biometrics at a distance. Challenges associated with iris acquisition systems stem largely from two requirements: (1) active NIR illumination and (2) pixel and spatial resolution. Firstly, standards dictate that NIR illumination be used as it has been found to emphasize iris textures while providing contrast between the iris and both the sclera and pupil. Secondly, the dimensions of the iris are such that iris recognition technologies require signification pixel and spatial resolutions in order to encapsulate enough biometric data. The Iris Image Interchange Format (ANSI INCITS 379-2004) specifies a lower limit of 100 pixels for the number of pixels across the iris region and sets a lower spatial resolution limit for two line pairs-per-mm at 60% contrast or higher.
Commercially available iris acquisition systems meet illumination and resolution requirements by imposing significant constraints on subjects during acquisition. Constraints imposed on parameters such as stand-off distance, capture volume, and subject motion account for some of the disadvantages of existing iris acquisition systems. Relaxation of these constraints cannot be achieved without a more demanding system design. For example, some system designs require subjects to pass through a portal type gate or by a glance-and-go type system during acquisition. These types of systems may provide larger stand-off distances (e.g., approximately 1-3 meters) and capture volumes (e.g., approximately 0.2 m×0.1 m×0.2 m). Such systems, however, are not scalable in terms of distance. In other words, it becomes more difficult to acquire iris images for biometric identification as the distance between the imaging system and the target increases. Furthermore, the constraints of commercially available iris acquisition systems prevent these systems from acquiring and tracking a person among multiple persons within a specified field of view from a distance. As a further disadvantage, such systems usually image one iris at a time and not two eyes simultaneously, or near simultaneously.
In addition, like other commercially available biometric systems, traditional iris acquisition systems are typically limited to a single biometric modality. In other words, such systems only acquire and process images of the iris. These single modal systems suffer from the limitations inherent in iris-only imaging systems and fail to take advantage of other techniques for biometric acquisition. In particular, although video surveillance technologies are also advancing to new levels of maturity, it has been observed that the technologies for identity tracking through biometrics and location tracking through intelligent video surveillance are evolving in isolation.